WASHINGTON, Jan 8 (Reuters) - The record cold U.S. temperatures may have a silver lining - killing off some tree-eating forest pests that have spread dangerously as the general climate warms up, scientists said.

The deep freeze that shattered decades-old records this week - causing fatalities and snarling air, road and rail traffic - could adversely affect pests such as the emerald ash bore, which is responsible for killing more than 10 million trees, said Robert Venette, a research biologist with the U.S. Forest Service in St. Paul, Minnesota.

Eighty percent of emerald ash bore may have been destroyed where temperatures fell to -22 to -26 Fahrenheit (-30 to 32 Celsius), he said.

"That's when they really start to freeze and die. And we've been seeing those kind of temperatures here in Minnesota, particularly in the Twin Cities and points north," Venette explained.

The emerald ash bore are spread over more than 20 states, from Colorado to Georgia, and have extended into Canada, he said, adding they are now in a larval state under the bark of infected trees.

Also affected will be gypsy moths, which have recently moved into northern Minnesota and eat the leaves of more than 300 species of trees, shrubs and plants, he said. The moths die at -17F (-27C).

"This cold weather should really have done a number on those populations," Venette said. Less severe cold for a prolonged period also can stress pests and kill them, he said.

The cold connected with the polar air mass that enveloped North America over the last several days has begun to ease up.

The National Weather Service said temperatures would start to moderate across the eastern two-thirds of the United States on Wednesday, but would still be 15 to 25 degrees below normal in the Midwest.

WOOLLY ADELGID SURVIVES

John Barnwell, director of forest policy with the Society of American Foresters, said the cold had not been severe or prolonged enough to kill the woolly adelgid, which has devastated hemlock forests in the eastern United States.

A negative side effect of severe cold is that it can damage forests by splitting or cracking limbs and stems, leaving them vulnerable to insects and other threats, Barnwell said.

The cold spell may not last long enough to affect most mountain pine beetles, which have devastated millions of acres (hectares) of forest in the United States and Canada, he said.

The Colorado State University extension service says in a fact sheet on the pest that midwinter temperatures of at least -30F (-34C) must be sustained for at least five days to kill larvae. The nation's low temperature on Wednesday was -33F (-36C) recorded at Crane Lake, Minnesota.

But the freeze may be a blessing for the Great Lakes as well, said Brenda Ekwurzel, a senior climate scientist at the Union of Concerned Scientists.

More ice cover could limit evaporation and help the lakes, which have seen shipping affected as their levels fall because of drought and other factors. (Reporting by Ian Simpson; editing by Gunna Dickson)